The present invention relates to a rear-projection screen and a rear-projection image display employing the rear-projection screen.
A rear-projection image display, such as a rear-projection television set, magnifies an image displayed on a small video source, such as a projection CRT (cathode-ray tube) or a liquid crystal display, by a projection lens and projects the image on a rear-projection screen. The image quality of the rear-projection image display has been greatly improved in recent years, and rear-projection image displays have come into wide use in the domestic and industrial fields.
When the rear-projection image display uses a projection CRT as a video source, an image of the three primary colors is composed on a rear-projection screen by combining red, green and blue images by the combined function of a CRT and a projection lens to display the image in a sufficiently high brightness on the rear-projection screen.
As stated in Japanese Patent Laid-open (Kokai) Nos. 56-117226 and 58-93043, such a rear-projection image display employs a two-layer laminated rear-projection screen consisting of a Fresnel lens sheet and a lenticular lens sheet. The lenticular lens sheet includes a diffusion material dispersed in particles in the lenticular sheet or formed in a diffusion layer over the surface of the lenticular lens.
FIG. 1 is a perspective view of an essential portion of a prior art rear-projection screen.
Referring to FIG. 1, a rear-projection screen 1 consists of a Fresnel lens sheet 2 and a lenticular lens sheet 3'. A video source, such as the screen of a CRT, is disposed behind the Fresnel lens sheet 2. The respective base sheets of the Fresnel lens sheet 2 and the lenticular lens sheet 3' are formed of a transparent, thermoplastic resin. Particles of a diffusion material 15 are dispersed in the base sheet of the lenticular lens sheet 3'. The Fresnel lens sheet 2 has an entrance surface 21 and an exit surface 22. The entrance surface 21 has the shape of a vertical arrangement of a plurality of horizontally elongate lenticular lenses each having the shape of a portion of a cylinder, and the exit surface 22 has the shape of the stepped setbacks of a Fresnel lens.
The lenticular lens sheet 3' has an entrance surface 31 formed by horizontally arranging a plurality of first vertically elongate lenticular lenses, and an exit surface 32 formed by horizontally arranging a plurality of second vertically elongate lenticular lenses substantially similar to the first vertically elongate lenticular lenses, and a plurality of ridges 33 formed between the adjacent second vertically elongate lenticular lenses, respectively. The upper surface of each ridge 33 is coated with a black, light absorbing strip 16.
Light rays emitted from points on an image displayed on the screen of a projection CRT travel through a projection lens, not shown, and fall on the entrance surface 21 of the Fresnel lens sheet 2. If the entrance surface 21 is a flat plane, the light rays emerging from the exit surface 22 of the Fresnel lens 2 are collimated substantially by the Fresnel lens forming the exit surface 22 of the Fresnel lens sheet 2, and then the substantially parallel light rays fall on the lenticular lens sheet 3'.
The parallel light rays are directed toward a focus near the second vertically elongate lenticular lenses forming the exit surface 32 by the first vertically elongate lenticular lenses forming the entrance surface 31, the light rays are diffused horizontally from the focus, the light rays are diffused vertically and horizontally by the particles of the diffusion material 15 dispersed in the base sheet of the lenticular lens sheet 3' and emerges from the surface of the lenticular lens sheet 3' on the image viewing side.
However, since the entrance surface 21 of the Fresnel lens sheet 2 is formed of the horizontally elongate lenticular lenses as shown in FIG. 1, the light rays falling on the entrance surface 21 of the Fresnel lens sheet 2 are diffused vertically by the horizontally elongate lenticular lenses, and the light rays are further diffused vertically by the particles of the diffusion material 15 dispersed in the base sheet of the lenticular lens sheet 3'.
The horizontally elongate lenticular lenses forming the entrance surface 21 of the Fresnel lens sheet 2 will be described in detail hereinafter.
Referring to FIG. 2 showing the Fresnel lens sheet 2 of the rear-projection screen 1 of FIG. 1 in a vertical sectional view, the entrance surface 21 of the Fresnel lens sheet 2 is formed, as mentioned above, by vertically arranging the plurality of horizontally elongate lenticular lenses at a pitch smaller than the pitch of scanning lines of the projected image or that of picture elements. The pitch of the lenticular lenses is determined so that moire resulting from interference between the lenticular lenses and the scanning lines and moire resulting from interference between the lenticular lenses and portions of the rings of the Fresnel lens of the Fresnel lens sheet 2, corresponding to the upper and lower portions of the screen are minimized. More concretely, the horizontally elongate lenticular lenses are arranged at a pitch far smaller than those of the first vertically elongate lenticular lenses and the second vertically elongate lenticular lenses of the lenticular lens sheet 3' and the pitch of the horizontally elongate lenticular lenses is determined so that the ratio between the pitch of the scanning lines and the pitch of the horizontally elongate lenticular lens is not the ratio of simple integers.
For example, if the rear-projection screen is 800 mm in horizontal size and 600 mm in vertical size, the horizontal pitch of the first vertically elongate lenticular lenses and the second vertically elongate lenticular lenses is 0.78 mm, and the number of horizontal scanning lines on the rear-projection screen is 480, the pitch of the scanning lines is 1.25 mm. In most cases, the pitch of the rings of the Fresnel lens is in the range of about 0.1 mm to about 0.12 mm, and the pitch of the horizontally elongate lenticular lenses is in the range of about 0.08 mm to about 0.1 mm, when the pitch of scanning lines is 1.25 mm.
When the incident light rays 14 fall on the horizontally elongate lenticular lenses forming the entrance surface 21, the angle of incidence of the light rays 14 for the same scanning line or the same picture element is dependent on the point of incidence. Accordingly, the light rays 14 are refracted at different angles of refraction, so that the incident light rays 14 are diffused vertically. Furthermore, if the radius of curvature of the horizontally elongate lenticular lens is relatively small, the incident angles of the incident light rays are relatively large and, consequently, the light rays 14 are diffused in a greater angular range and the vertical viewing angle increases.
The vertically elongate lenticular lenses forming the entrance surface 31 and exit surface 32 of the lenticular lens sheet 3' will be described in detail hereinafter.
Referring to FIGS. 3 and 4 showing the lenticular lens sheet 3' of the rear-projection screen 1 of FIG. 1 in horizontal sectional views, the surface of each first vertically elongate lenticular lens forming the entrance surface 31 (FIG. 1) is a portion of the surface of an elliptic cylinder of a cross section having the shape of an ellipse having a major axis extending in the direction of the thickness of the lenticular lens sheet 3' i.e., along a line 1--1' having one focus positioned within the base sheet and the other focus positioned near the exit surface 32. The eccentricity e of the ellipse is approximately equal to the reciprocal of the refractive index n of the base sheet.
As shown in FIG. 3, all the green rays falling on the first vertically elongate lenticular lens in parallel to the major axis of the ellipse converge on the focus positioned near the exit surface 32, and then the green rays diverge horizontally from the focus. As shown in FIG. 4, all the red and blue rays falling on the first vertically elongate lenticular lens at an angle to the major axis of the ellipse converge on the focus positioned on the exit surface 32, and then the red and blue rays diverge horizontally from the focus.
The surface of each second vertically elongate lenticular lens forming the exit surface 32 is a portion of the surface of an elliptic cylinder substantially resembling the mirror image of the surface of the corresponding first vertically elongate lenticular lens. The second vertically elongate lenticular lens makes the horizontal directional characteristics of the emerging red, green and blue rays parallel to each other.
The light diffusion material 15 dispersed in the base sheet of the lenticular lens sheet 3', i.e., a lenticular lens sheet 3' containing dispersed particles of the light diffusing material 15, will be described in detail hereinafter.
Referring to FIGS. 5a and 5b showing the lenticular lens sheet 3' of the rear-projection screen 1 of FIG. 1 in a vertical sectional view and a horizontal sectional view, respectively, particles of the light diffusing material 15 are dispersed in the base sheet of the lenticular lens sheet 3' to diffuse the incident light rays 14 vertically and horizontally while the incident light rays 14 travel from the entrance surface 31 to the exit surface 32. The angular range of diffusion of the light rays 14 increases, the directional characteristics are widened and the viewing angle increases with the increase of the quantity of the light diffusing material 15 contained in the base sheet of the lenticular lens sheet 3'.
Some problems in the foregoing prior art transparent screen TS to be solved will be described hereinafter.
First problem is insufficient range in the vertical viewing angle and the horizontal viewing angle.
Referring to FIG. 6, the vertical viewing angle and the horizontal viewing angle are 0.degree. when the viewing position is on a line FD, i.e., a perpendicular at an image point on the rear-projection screen TS. The brightness of the image point on the rear-projection screen TS as viewed from the viewing position on the line FD is B.sub.0, and the brightness of the image point as viewed from a viewing position on a line extending at a horizontal angle .alpha. is B.sub..alpha.. Then, the relative brightness H=B.sub..alpha. /B.sub.0. Similarly, the brightness of the image point as viewed from a viewing position on a line extending at a vertical viewing angle .beta. is B.sub..beta. and the relative brightness H=B.sub..beta./B.sub.0.
If the relative brightness H is smaller than a threshold, the image point is scarcely visible. The ranges of the horizontal viewing angle .alpha. and the vertical viewing angle .beta., in which the image point is visible, will be referred to as a horizontal angular range HR of visibility and a vertical angular range VR of visibility, respectively. The horizontal viewing angle .alpha. and the vertical viewing angle .beta. making the relative brightness H=B.sub..alpha. /B.sub.0 =50% and the relative brightness H=B.sub..beta. /B.sub.0 =50% will be referred to as a specific horizontal viewing angle and a specific vertical viewing angle.
FIG. 7 is a graph showing the directional characteristics of the prior art rear-projection screen, in which the horizontal viewing angle .alpha. and the vertical viewing angle .beta. are measured on the horizontal axis, a curve indicated by continuous lines represents horizontal directional characteristics, and a curve indicated by broken lines represents vertical directional characteristics.
As shown in FIG. 7, the image point on the rear-projection screen is invisible when the horizontal viewing angle .alpha. is outside an angular range of .+-.47.degree. or when the vertical viewing angle .beta. is outside an angular range of .+-.25.degree.. The specific vertical viewing angle, at which the relative brightness H=B.sub..beta. /B.sub.0 =50%, is as small as on the order of .+-.9.degree..
FIG. 8 is a graph showing the vertical directional characteristics of the horizontally elongate lenticular lens of the Fresnel lens sheet 2 of the prior art rear-projection screen.
As shown in FIG. 8, the prior art rear-projection screen having the Fresnel lens sheet 2 provided with horizontally elongate lenticular lenses having a radius of curvature on the order of 0.3 mm has a vertical angular range of visibility on the order of .+-.4.degree.. The combined functions of the horizontally elongate lenticular lenses and the light diffusing material dispersed in the base sheet of the lenticular lens sheet 3 makes the vertical angular range of visibility of the rear-projection screen .+-.25.degree..
FIG. 9 is a graph showing desirable directional characteristics of a rear-projection screen.
As shown in FIG. 9, the vertical directional characteristics and the horizontal directional characteristics are wider than those of the prior art rear-projection screen, and both the desirable horizontal angular range of visibility and the desirable vertical angular range of visibility are on the order of .+-.70.degree..
The expansion of the vertical directional characteristics and increase in the specific vertical viewing angle to improve the directional characteristics of the prior art rear-projection screen can be achieved by increasing the quantity of the light diffusing material 15 contained in the base sheet of the lenticular lens sheet 3' or reducing the radius of curvature of the horizontally elongate lenticular lenses forming the entrance surface 21 of the Fresnel lens sheet 2.
However, increase in the quantity of the light diffusing material 15 contained in the base sheet of the lenticular lens sheet 3' of the prior art rear-projection screen entails the following problems.
FIG. 10 is a vertical sectional view of the rear-projection screen of FIG. 1, showing the vertical diffusion of the incident light rays 14.
As shown in FIG. 10, the incident light rays 14 falling on the Fresnel lens sheet 2 are refracted and diffused vertically by the horizontally elongate lenticular lenses forming the entrance surface 21. The diffused light rays fall on the lenticular lens sheet 3'. Since the diffused light rays are further diffused by the light diffusing material 15 contained in the base sheet of the lenticular lens sheet 3' as the diffused light rays travel through the lenticular lens sheet 3', the width d of the light rays outgoing from the lenticular lens sheet 3' is greater than the width of the incident light rays 14, and the width of the scanning line or the size of the picture element is increased on the exit surface 32 and, consequently, the image focusing characteristics is deteriorated.
If the quantity of the light diffusing material 15 contained in the base sheet of the lenticular lens sheet 3' is increased to enhance the vertical directional characteristics of the rear-projection screen, the width of the scanning line or the size of the picture element will be further increased on the exit surface 32 of the lenticular lens sheet 3' and, consequently, the image focusing characteristics will be further deteriorated.
As shown in FIGS. 5a, 5b and 10, the incident light rays 14 undergo not only diffusion but also scattering in the lenticular lens sheet 3' containing the light diffusing material 15. Therefore, some of the light rays 14 are reflected toward the entrance surface 31 (FIG. 10), stray in the lenticular lens sheet 3' or are absorbed by the light absorbing strips 16. Such light rays are unable to reach the foci positioned near the exit surface 32 and unable to emerge from the exit surface 32, which reduces the brightness of the rear-projection screen. The degree of reduction in the brightness increases with increase in the light diffusing material content of the base sheet of the lenticular lens sheet 3'.
The light rays among the incident light rays 14, scattered by the light diffusing material 15 and strayed in the lenticular lens sheet 3' are unnecessarily reflected repeatedly in the projecting optical system and some of them finally reach the exit surface 32 of the lenticular lens sheet 3' to deteriorate the contrast of the image. Although nearly half of the ambient light, such as illuminating light, is absorbed by the light absorbing strips 16 formed on the exit surface 32 of the lenticular lens sheet 3' the ambient light falling on the second vertically elongate lenticular lenses forming the exit surface 32 is reflected diffusibly by the light diffusing material 15, which also deteriorates contrast in the image. Thus, the effect of the light diffusing material 15 on the deterioration of contrast is significant.
Reduction in the radius of curvature of the horizontally elongate lenticular lenses forming the entrance surface 21 of the Fresnel lens sheet 2 of the prior art rear-projection screen of FIG. 1 entails the following problems.
As shown in FIG. 10, the incident light rays 14 falling on the Fresnel lens sheet 2 are refracted and diffused vertically by the horizontally elongate lenticular lenses forming the entrance surface 21, the vertically diffused light rays fall on the lenticular lens sheet 3' and the vertically diffused light rays are further diffused by the light diffusing material 15 dispersed in the base sheet of the lenticular lens sheet 3'. Accordingly, the width d of the light rays on the exit surface 32 of the lenticular lens sheet 3' is greater than that of the incident light rays 14 on the entrance surface 21 of the Fresnel lens sheet 2. Consequently, the width of a scanning line or the size of a picture element is increased on the exit surface 32 to deteriorate the image focusing characteristics.
If the radius of curvature of the horizontally lenticular lenses forming the entrance surface 21 of the Fresnel lens sheet 2 is reduced to enhance the vertical directional characteristics, the width of a scanning line or the size of a picture element on the exit surface 32 of the lenticular lens sheet 3' is further increased.
Thus, it is impossible to enhance the vertical directional characteristics of the prior art rear-projection screen without deteriorating the image focusing characteristics, brightness and contrast.
Second problem is the reduction of color shift. Color shift is the variation of the color of an image according to the horizontal viewing angle .alpha. resulting from change in the color balance of the three primary colors, i.e., red, green and blue, due to slight difference between the respective directional characteristics of the three primary colors when the red, green and blue light rays are diffused horizontally by the lenticular lens sheet 3, details of which will be given later.
As mentioned above, the surfaces of the second vertically elongate lenticular lenses forming the exit surface 32 of the lenticular lens sheet 3 make the incident red, green and blue rays substantially parallel to each other. Such a function of the vertically elongate lenticular lenses reduces color shift to some extent. However, the effect of this function is not necessarily satisfactory.
FIG. 11 is a graph showing the directional characteristics of the lenticular lens sheet 3' of FIG. 4 for red rays and blue rays.
As shown in FIG. 11, the difference in relative brightness between red rays and blue rays for horizontal viewing angle .alpha.=45.degree. is 50% and such a large difference causes color shift. Accordingly, the difference in relative brightness between red rays and blue rays needs to be reduced greatly.
Thus, the degree of color shift with the prior art rear-projection screen is not small enough.
Third problem is the minimization of moire. As mentioned above, efforts are made in determining the dimensions of the components of the prior art rear-projection screen relating with moire, including the pitch of the horizontally elongate lenticular lenses so that moire may be minimized. However, the effect of the efforts on minimizing moire is not necessarily satisfactory, because horizontal bright lines and horizontal dark lines are formed alternately on the exit surface 22 of the Fresnel lens sheet 2 of the rear-projection screen due to the light focusing characteristics of the horizontally elongate lenticular lenses, the details of which will be given later.
Thus, the prior art rear-projection screen is unable to minimize moire satisfactorily to a small extent.